CONSTRAINED MODEL PREDICTIVE CONTROL FOR A PILOT HYDROTREATING PLANT H. M. S. LABABIDI  , I. M. ALATIQI and Y. I. ALI Department of Chemical Engineering, College of Engineering and Petroleum, Kuwait University, Safat, Kuwait T he main goal of this work is to investigate the application of constrained model predictive control (CMPC) for a pilot hydrotreating plant. The pilot plant used in this study is a catalyst testing unit operated for desulphurization of atmospheric residue feedstock and using the same catalyst and operating conditions of actual refinery hydrotreating processes. The operational goal in such reactors is to maintain the sulphur content of the product at a desired limit. Degree of desulphurization is highly sensitive to reaction temperature and is affected by a number of parameters such as H 2 /oil ratio and catalyst deactivation. A CMPC controller has been experimentally implemented to optimize the reaction zone temperatures that would result in achieving the desired degree of desulphur- ization. Selected manipulated variables are system pressure and three reactor skin tempera- tures. A significant improvement in controller performance is obtained with the CMPC strategy, when compared with the existing PID-based control structure. This study highlighted the main factors affecting the operation and dynamics of such processes. The pilot plant can be considered as a test bed for implementing online optimal control and nonlinear control strategies for the catalytic hydrotreating units in refineries. Keywords: constrained model predictive control (CMPC); advanced control; catalytic reactors; pilot plant. INTRODUCTION An increasing number of refineries are revamping major processes in an effort to combat declining profit margins caused by crude prices and increased competition. While refiners direct their efforts towards obtaining more from the proverbial ‘bottom of the barrel’, they are also being targeted for increased scrutiny by environmental regulatory agencies. One of the challenging problems is reducing SO X emissions and producing low sulphur products. Owing to the fact that Kuwaiti crude is high in sulphur, sulphur removal processes are extremely important for local refineries as well as worldwide refineries processing Kuwaiti crude. In particular, residue hydrodesulphurization processes are considered essential and beneficial for upgrad- ing residuum and meeting the environmental regulations. As an alternative to process revamp through changes in the processing units, refiners are currently exploring the benefits of advanced process control strategies more than ever. The trend for installing online computer control in refineries has intensified during the past few years. Process complexities, nonlinearity, and long time delays have always characterized most processes encountered in the refining and process industries. Advanced and multi- variable predictive controls became the focus of research to deal with such difficulties. The pioneering work of Richalet et al. (1978), which demonstrated the use of IDCOM for an FCCU distillation column, and a steam gen- erator, ushered in a new wave of application developments and field trials in refining, process and steam generation industries. An alternative multivariable approach to IDCOM using dynamic matrix representation and known as dynamic matrix control (DMC) was developed by Cutler and Ramaker (1979), and Cutler (1983). Yet another alternative was developed independently by Martin (1981) in the form of a long-range predictive control, which was later field-tested in several process units. Following these beginnings which could be characterized as pilot appli- cations of advanced and multivariable control, advanced process control started its penetration of industrial plants, driven by real process operation difficulties, and motivated by promises of economic benefits. Field application of advanced control in refinery operation was reported by several investigators, including Agnihotri et al. (1987) and Al-Abdeen et al. (2000) on hydrocracker units, Grosdidier and Kennedy (1990) on fractionators, Van Wijk and Pope (1992) and Paraschiv et al. (1996) on atmospheric crude  Correspondence to: Dr. H.M.S. Lababidi, Department of Chemical Engineering, College of Engineering and Petroleum, Kuwait University, PO Box 5969, Safat, 13060 Kuwait. E-mail:lababidi@eng.kuniv.edu.kw 1293 0263–8762/04/$30.00+0.00 # 2004 Institution of Chemical Engineers Trans IChemE, Part A, October 2004 Chemical Engineering Research and Design, 82(A10): 1293–1304